The electrical spraying of liquids to produce ions, termed electrospray, was first used as a tool in analytical chemistry in the late 1960's. This initial work was not expanded upon until 1984, when John Fenn and co-workers reported combining electrospray ionization and mass spectrometry (ESI/MS) to analyze high molecular weight species. See, e.g., U.S. Pat. No. 4,542,293 to J. Fenn et al.; U.S. Pat. No. 4,531,056 to M. Labowsky et al.
A major limitation of current electrospray technology is that the electrospray is performed at atmospheric pressure. See, e.g., U.S. Pat. No. 4,977,320 to Chowdhury et al.; U.S. Pat. No. 4,935,624 to Henion et al.; U.S. Pat. No. 4,885,076 to Smith et al.; U.S. Pat. No. 4,842,701 to Smith et al. This creates a need for differential pumping and reduces sampling efficiency. See M. Ikonomou et al., Anal. Chem. 62, 957 (1990); M. Mann, Organ. Mass Spec. 25, 575 (1990). A further problem is that high flow rates (e.g., above 0.5 .mu.l/minute) have been required. R. Smith et al., Anal. Chem. 60, 1948 (1988).
In view of the foregoing, objects of the present invention are to provide a way to electrospray directly into a vacuum, and a way to electrospray at low flow rates.